In an LTE (Long Term Evolution) system, data may be sent by means of channel sharing. For uplink data, an eNB (evolved Node Bastation, or evolved NodeB) may allocate an uplink transmission resource to user equipment. Only a user equipment (UE) that receives an uplink scheduling grant can send uplink data. That is, a UE cannot send data if the UE does not receive an uplink scheduling grant.
3GPP (3rd Generation Partnership Project) has introduced an LAA (License Assisted Access, licensed-assisted access or non-licensed spectrum) cell. This cell uses a non-licensed spectrum.
An LAA cell uses a non-licensed spectrum. When data transmission is performed in the LAA cell, a data transmission resource may no longer be obtained by using an eNB scheduling scheme. Instead, the data transmission resource may be obtained by means of channel preemption. That is, a base station that serves the LAA cell performs energy detection on a channel that is corresponding to the non-licensed spectrum, to determine whether the channel is idle. If the channel is idle, the LAA cell preempts the channel. In this case, the channel can only be used for data transmission in the LAA cell for a given period of time, and after this period of time, the LAA cell releases the channel. Once the LAA cell preempts the channel, the base station that serves the LAA cell informs UEs in the LAA cell about the preemption.
When a data transmission resource of an LAA cell is obtained by means of channel preemption, and a channel can only be preempted for a period of time, a PRACH (Physical Random Access Channel,) time-frequency resource allocated by an eNB during a process in which a UE randomly accesses the LAA cell may become unavailable due to that the LAA cell fails to preempt a channel or a period of time in which a preempted channel is allowed for use has expired.